1. Field of the Invention
The present invention relates to a slide fastener stringer including a stringer tape and a series of fastener coupling elements sewn to the tape along one longitudinal edge thereof by utilizing multi-thread chain stitch or "double locked stitch".
2. Prior Art
In sewing a series of fastener coupling elements to a stringer tape for a slide fastener, one of the most widely used stitch types is multi-thread chain stitch or "double locked stitch", which is formed with two or more sewing threads, i.e. needle and looper threads. It has been customary to use a spun or multifilament yarn for either the needle thread or the looper thread, for such non-monofilament yarns are flexible and less stretchable and hence enable the fastener elements to be sewn to the tape tightly on a high-speed sewing machine without breakage of a sewing needle.
A common problem encountered with such prior art slide fastener stringer is that, because the material and fabric structure of modern stringer tapes are usually of the type having less frictional resistance, the sewing threads are liable to become loose from its cut end portions which have been cut as the fastener stringer of a continuous length has been severed into a slide fastener length. With this arrangement, when the opposed stringer tapes are laterally pulled at their one end in opposite directions during threading of a pair of the interengaged fastener stringers through a slider, the extreme one or two or even more of the fastener elements on each tape would be easily displaced. Consequently, it would be difficult or sometimes impossible to mount the slider onto the interengaged fastener stringer.
U.S. Pat. No. 3,783,476 discloses a slide fastener stringer having a row of fastener elements secured to a stringer tape by means of single-needle double locked stitch formed with needle and looper threads, of which only needle thread includes a monofilament yarn. The needle thread is disposed on the fastener-element side of the slide fastener stringer, and hence, the stitching must be done from that side. This requires a specially designed guide means to support the slide fastener stringer such that the surface of the stringer tape on which the fastener elements are to be attached faces upwardly during sewing operation. With this arrangement, sufficient degree of tightness of the stitching is difficult to achieve. Further, because the fastener elements are held, against the stringer tape, by the monofilamentary needle thread and both the fastener elements and the needle thread have less frictional resistance, the needle thread is liable to slip on the fastener elements and hence is not likely to keep the fastener elements stably in position on the tape. Moreover, the looper thread projects from the tape surface so that a slider is likely to wear out the looper thread.
U.S. Pat. No. 3,768,125 discloses another slide fastener stringer having a row of fastener elements secured to a stringer tape by means of single-needle double locked stitch formed with needle and looper threads. Such prior art slide fastener stringer has not sufficient degree of flexibility, which is one of the most important factors for slide fasteners, partly because monofilament yarns have rigidity by nature and partly because loops of the monofilamentary looper thread extend across and over the fastener elements. Further, because the fastener elements are held, against the stringer tape, by the monofilamentary looper thread and both the fastener elements and the monofilamentary looper thread have less frictional resistance, the needle thread is liable to slip on the fastener elements and hence is not likely to keep the fastener elements stably in position on the tape.